Imaging the planet around GJ 504

Figure 2 from Skemer, Morley, Zimmerman, et al.: De-rotated co-add of all photometric quality GJ 504 images taken in the LNB7 (3.95 μm) filter. The planet, GJ 504 b, is the circled point source to the northwest of the star, at separation ∼2.′′5. Due to the large dynamic range of structure in the image, two regions are displayed with different flux scales: (1) within 2′′ of the star, the Airy rings are displayed with a logarithmic stretch; the outer region (2) that includes the planet is displayed with a linear stretch centered at zero. The corresponding grayscale bars on the right-hand side indicate the flux in units normalized to the peak of the unsaturated star’s PSF.

Andy Skemer, Caroline Morley, myself, and the rest of the LEECH Science Team posted a paper on the arXiv today: Characterization of the Coldest Directly Imaged Exoplanet, GJ 504 b, and Evidence for Super-Stellar Metallicity.

The article, soon to be published in ApJ, describes new observations of the planet candidate discovered to orbit GJ 504 a few years ago (Kuzuhara et al., 2013). By measuring the thermal emission from the companion in three custom filters in the range 3.6–4.1 microns, we’ve made new inferences about the temperature, density, and composition of the atmosphere. The observations were made possible with LMIRCam, an AO-fed infrared camera on the Large Binocular Telescope.

I was responsible for the entirety of the data reduction, taking steps familiar to other high-contrast observers in this wavelength regime:

  • Aligning and combing thousands of raw, short exposures, correcting for background sky noise and detector pattern noise
  • Angular differential imaging to subtract the star (here implemented with the KLIP algorithm)
  • Contrast curve calculation, PSF modeling, photometry, and error analysis

GJ 504 b is on the extreme faint end of all the planetary-mass companions imaged to date (as Figure 4 in the journal article illustrates). Fortunately, the combination of relatively high angular separation (2.5 arcsec from the host star) with the deep sensitivity of LMIRCam–as shown above in the co-add image–was high enough to secure a detection at each filter. The wavefront correction provided by the LBTI AO system was such that Airy rings spanning three orders of magnitude in intensity are evident, out beyond a 1-arcsec radius. Even without PSF subtraction, the planet is evident at a contrast of 1E-5, in the northwest corner of the co-add image.

There is a lot more we could learn from the infrared SED at wavelengths redward of 4 microns. For such a faint object, however, that part of the spectrum is not likely to be accessed from the ground. Therefore, in the future GJ 504 will make an excellent target for JWST NIRCam.